Atomic History and Atomic History and TheoriesTheories

AtomAtom

• Definition: Definition: the smallest the smallest particle of particle of any element any element that retains that retains the the properties of properties of that element.that element.

Practice QuizPractice Quiz

• What is the smallest particle of What is the smallest particle of magnesium that retains the magnesium that retains the properties of magnesium?properties of magnesium?

Electron Electron MicroscopesMicroscopes

• With the use of tunneling electron With the use of tunneling electron microscopes we can sort of “see” atomsmicroscopes we can sort of “see” atoms

Image of copper atoms

Image of silicon atoms

These electron These electron microscopes were microscopes were

invented in the mid-invented in the mid-1900’s. How did 1900’s. How did

scientists figure out scientists figure out atomic structure atomic structure

without being able to without being able to look at any atoms?look at any atoms?

The History of The History of Atomic TheoryAtomic Theory

But But first….THEORYfirst….THEORY

• What is a What is a theorytheory in science? in science?

• A theory is a well-tested A theory is a well-tested explanationexplanation of what happens in nature.of what happens in nature.

• In layman’s terms, if something is In layman’s terms, if something is said to be “just a theory,” it usually said to be “just a theory,” it usually means that it is a mere guess, or is means that it is a mere guess, or is unproven. But in scientific terms, a unproven. But in scientific terms, a theory theory implies that something has implies that something has been well-tested and verified many been well-tested and verified many times by multiple groups of times by multiple groups of scientists.scientists.

But first….LawBut first….Law• A scientific A scientific lawlaw is a statement of is a statement of

something that seems to be true in something that seems to be true in the natural world, but doesn’t the natural world, but doesn’t explain why.explain why.

• Example: The Example: The LawLaw of Gravity. of Gravity. Newton could use this law to predict Newton could use this law to predict the behavior of a dropped object, the behavior of a dropped object, but he couldn't explain why it but he couldn't explain why it happened. happened.

Remember!Remember!

• A LAW describes what happens A LAW describes what happens in nature, but doesn’t explain in nature, but doesn’t explain why.why.

• A THEORY is an explanation of A THEORY is an explanation of what happens in nature based what happens in nature based on repeated observations or on repeated observations or experiments.experiments.

The History of The History of Atomic TheoryAtomic Theory

DemocritusDemocritus

• Greek philosopher, not scientistGreek philosopher, not scientist

• Proposed fourth century B.C.E. Proposed fourth century B.C.E. (Before Common Era)(Before Common Era)

• first to suggest that matter is made first to suggest that matter is made of tiny particles called atoms of tiny particles called atoms “atomos”“atomos”

• ““atomos” = Greek for “indivisible”atomos” = Greek for “indivisible”

• ideas were rejected by Aristotle, ideas were rejected by Aristotle, who was very influential, and who was very influential, and therefore forgotten for two therefore forgotten for two thousand years.thousand years.

Democritus’ atomic model:Democritus’ atomic model:atoms were small, hard atoms were small, hard

particles that were different particles that were different shapes and sizesshapes and sizes

John DaltonJohn Dalton

• First to propose an ATOMIC THEORYFirst to propose an ATOMIC THEORY based on science: based on science:1. Each element is composed of small 1. Each element is composed of small particles called atoms.particles called atoms.2. All atoms of a given element are 2. All atoms of a given element are identical*; the atoms of different identical*; the atoms of different elements are different.elements are different.3. Atoms are not created or destroyed 3. Atoms are not created or destroyed in chemical reactions.in chemical reactions.4. Compounds are formed when atoms 4. Compounds are formed when atoms of more than one element combine.of more than one element combine.

Dalton’s atomic model:Dalton’s atomic model:a tiny sphere that is a tiny sphere that is

indivisibleindivisible

1.1.All matter is composed of All matter is composed of extremely small particles extremely small particles called atomscalled atoms

2.2. Atoms of a given element Atoms of a given element are identical in size, mass, are identical in size, mass, and other properties; atoms and other properties; atoms of different elements differ in of different elements differ in size, mass, & other size, mass, & other propertiesproperties

ELEMENT2

ELEMENT2

ELEMENT 3

ELEMENT 3

ELEMENT 1

ELEMENT 1

ELEMENT4

ELEMENT4

Atomic TheoryAtomic Theory

3.3. Atoms cannot be Atoms cannot be created or destroyedcreated or destroyed

4.4. Atoms of different Atoms of different elements combine to elements combine to form compoundsform compounds

5.5. In chemical rxns, In chemical rxns, atoms are combined, atoms are combined, separated, or separated, or rearrangedrearranged

++++ ++++

Atomic TheoryAtomic Theory

J.J. ThomsonJ.J. Thomson

• First to suggest that there were First to suggest that there were particles smaller than the atom.particles smaller than the atom.

• Discovered the first subatmic Discovered the first subatmic particle…….. the ELECTRON!particle…….. the ELECTRON!

• Developed the “plum pudding Developed the “plum pudding model.”model.”

Thomson’s “plum-pudding” Thomson’s “plum-pudding” model:model:

the atom is composed of electrons the atom is composed of electrons surrounded by a cloud of positive surrounded by a cloud of positive charge to balance the electron's charge to balance the electron's

negative chargenegative charge

Thompson vs. Thompson vs. DaltonDalton

• Thompsons model proved Thompsons model proved Daltons Atomic Theory was Daltons Atomic Theory was wrong….wrong….• Atoms are divisible into smaller Atoms are divisible into smaller

subatomic particlessubatomic particles

Remember!Remember!

Ernest Ernest RutherfordRutherford

• gold foil experimentgold foil experiment

• discovered the NUCLEUS, and realized discovered the NUCLEUS, and realized that it was very dense and positively that it was very dense and positively chargedcharged

• suggested that most of the atom is suggested that most of the atom is empty spaceempty space

Gold Foil Gold Foil ExperimentExperiment::

Rutherford shot tiny Rutherford shot tiny positively-charged alpha positively-charged alpha

particles through a thin sheet particles through a thin sheet of gold foilof gold foil

What Rutherford expected:

What really happened:

Gold Foil ExperimentGold Foil Experiment::Some of the alpha particles Some of the alpha particles

bounced back.bounced back.

What Rutherford expected:

What really happened:

Such huge deflections could Such huge deflections could mean only one thing… mean only one thing… some of the alpha particles some of the alpha particles had run into massive had run into massive concentrations of positive concentrations of positive charge and, since like charge and, since like charges repel, they had charges repel, they had been hurled straight back been hurled straight back by them. by them.

Rutherford’s atomic model:Rutherford’s atomic model:All of the positive charge is crammed All of the positive charge is crammed

inside a tiny, massive nucleus about ten inside a tiny, massive nucleus about ten thousand times smaller than the atom thousand times smaller than the atom as a whole. The atom is mostly empty as a whole. The atom is mostly empty

space. space.

Animation of Animation of Rutherford’s Rutherford’s experimentexperiment

• http://www.mhhe.com/http://www.mhhe.com/physsci/chemistry/physsci/chemistry/

essentialchemistry/flash/essentialchemistry/flash/ruther14.swfruther14.swf

Rutherford’s Rutherford’s ConclusionConclusion

• Eight years after the Gold Foil Eight years after the Gold Foil ExperimentExperiment

• Nucleus contains PROTONS, Nucleus contains PROTONS, positively charged particlespositively charged particles

Scientists were looking Scientists were looking for missing mass. for missing mass.

The protons and The protons and electrons in the atom electrons in the atom didn’t weigh enough didn’t weigh enough

to account for the to account for the mass of the atommass of the atom

James ChadwickJames Chadwick

• Rutherford’s co-workerRutherford’s co-worker

• proved the existence ofproved the existence of

neutrons, neutral particle in the neutrons, neutral particle in the atom’s nucleus.atom’s nucleus.

Chadwick’s model Chadwick’s model of the atomof the atom

Protons and neutrons are in the nucleus, which only takes up one millionth of a billionth of the volume of the atom. The electrons arein the electron cloud, which constitutes almost all of the volume of the atom.

In fact, if an atom were In fact, if an atom were expanded to the size of a expanded to the size of a

cathedral,cathedral,

the nucleus would be about the size of a housefly!

Subatomic Subatomic Particles to KnowParticles to Know

Subatomic

Particle

Location Mass Charge

Proton Nucleus 1 amu +1

Neutron Nucleus 1 amu 0

Electron Electron

Cloud

Practically

zero

-1